The present invention relates to a charge-eliminating apparatus which is used for eliminating electric charge on a photoreceptor drum of an image forming apparatus such as an electrostatic copier.
An image forming apparatus such as an electrostatic copier is composed in such a manner that: a charged photoreceptor (a photoreceptor drum will be explained hereafter as an example of the photoreceptor) is exposed to light according to the document information so that an electrostatic latent image can be formed; the electrostatic latent image is developed by toner so that it can be visualized; and the obtained toner image is transferred onto a transfer paper and fixed. Recently, this kind of image forming apparatus has been used in all industrial fields.
In order to prevent toner from adhering to a non-image zone which is a zone out of a region corresponding to an original document and an unnecessary pixel zone on the photoreceptor drum surface, a charge-eliminating apparatus is provided to the above kind of image forming apparatus to eliminate charge from the non-image zone and the unnecessary pixel zone on the photoreceptor drum surface by irradiating such zones. The charge-eliminating apparatus is composed of a plurality of light emitting elements and controlled to turn them on and off in accordance with a size of a document zone. In this case, light emitting diodes (which will be called LED hereafter) are generally used as the light emitting elements.
FIG. 15 is a side view which shows the composition of an image forming apparatus around a conventional charge-eliminating apparatus. The numeral 1 is a photoreceptor drum which composes an image carrier. The numeral 2 is an electrode which charges the photoreceptor drum 1 by corona discharge. The numeral 3 is a charge-eliminating apparatus which eliminates charge from the non-image zone on the photoreceptor drum 1. In the charge-eliminating apparatus 3, there are arranged a plurality of LEDs in the direction vertical to the drawing surface.
FIG. 16 is a schematic illustration which shows the composition of the charge-eliminating apparatus 3 in detail. The numeral 3a is a print substrate or a printed plate board on which the parts of the charge-eliminating apparatus are provided. The numeral 3b is a driver IC which drives a light source selectively. The numeral 3c is a lamp house which holds a plurality of LEDs composing the light source, and which separates a light flux emitted from one of the LEDs from another light flux emitted from the other one of the LEDs. The numeral 3d is the light source composed of a plurality of LEDs which are held by the lamp house 3c.
Recently there is now an increased demand for compact copiers. In accordance with the demand, the size of a photoreceptor drum which occupies a pretty wide space in a copier, has been reduced lately.
However, various kinds of parts are placed very closely around the photoreceptor drum of a copier. Further, it is anticipated that when the size of a photoreceptor drum is reduced, the parts are further crowded around the the photoreceptor drum surface. Consequently, the size of the charge-eliminating apparatus must be reduced.
However, in the case of a conventional charge-eliminating apparatus which emits light perpendicularly to the print substrate, the space to install the LED holding substrate is needed, so that it is difficult to reduce the size of the apparatus.
Consequently, even when there is a demand to use a photoreceptor drum of a small diameter, the size of the drum is limited by the size of a charge-eliminating apparatus, so that the image forming apparatus of a small size can not be realized.
It can be considered to use the bare chip type LED in order to reduce the size of a charge-eliminating apparatus. However, there is caused a problem even in this case.
FIG. 17 is a view of a charge-eliminating apparatus in which a bare chip LED is used, wherein the view is taken in the direction of emitted light. In the case of this drawing, 0.4 mm square bare chip LEDs are used. These bare chip LEDs are placed on an electrode at the cathode of a print substrate, and the anode side of the LEDs is connected with an electrode at the anode side of the print substrate through a wire (the electrodes on the print substrate are omitted in the drawing). When this type of charge-eliminating apparatus is manufactured, the lamp house 3c is mounted after the bare chip LEDs are provided to the print substrate by means of wire bonding. When the lamp house 3c is mounted, it should not come into contact with the wire of the bare chip LED, so that the size of the opening portion or window portion of the lamp house can not be reduced.
The problem is caused is that since the shape of the photoreceptor drum is cylindrical, the boundary of the charge-eliminating zone becomes vague. Namely, as illustrated in FIG. 18, the irradiating angle of the light emitted from a LED is extended, so that the incident angle upon the drum surface becomes large. As a result, the contour of the image zone and the non-image zone becomes vague, which can be a problem when a binding margin is required (the binding margin mode). FIG. 19 is a schematic illustration which shows the result of the binding margin copy operation. Charge-eliminating zone A lies at edges of the transfer paper and at the central portion of the transfer paper with regard to the direction (which is called the main-scanning direction) perpendicular to the drum rotating direction. The zones except this charge-eliminating zone A are charging zones (the image forming zone) B and C. In this case, image blurring zone D in the main-scanning direction is caused between the charge-eliminating zone and charging zone because of the aforementioned relation between the drum shape and the extended radiating angle.
Although not shown in FIG. 19, such image blurring zone D, of course, also takes place in the sub-scanning direction. This blurring zone D can be the cause of image deterioration.
The light amount distribution given to the surface of the photoreceptor drum 1 by the charge-eliminating apparatus 3 is shown in FIG. 20.
This graph shows the light amount distribution when the photoreceptor drum 1 is irradiated by six LEDs of A to F. The surface facing the front of a LED generally receives a large amount of light and the surface facing the position located between LEDs generally receives a small amount of light. According to the result of measurement, the difference of the amount of light between the above-described two surfaces is larger than 10%.
In this case, the apparatus is adjusted in such a manner that the minimum amount indicated as MIN of light emitted from LEDs can be the minimum amount of light which is necessary for eliminating charge. In this way charge-elimination can be conducted in all zones required to eliminate charge thereon. If charge-elimination is not carried out in some zones, there will be caused such problems that: the toner consumption is increased; and the image quality is deteriorated.
Furthermore, the excessively large amount of light is given to the portions on the photoreceptor drum surface except the portions to which the minimum amount of light is given, so that the fatigue of the photoreceptor drum 1 advances locally and its life is shortened. Further the power consumption of the charge-eliminating apparatus is uneconomically increased. Furthermore, the life of the charge-eliminating apparatus is shortened.
Even though there are the problems described above, the occurrence of the zones which are not charge-eliminated can not be permitted, so that the amount of light emitted from the charge-eliminating apparatus 3 is set at a large value which is sufficient for charge-elimination.